1. Field of the Invention
The invention relates to a process for improving the adhesion of a layer, made of a material that is crosslinkable by exposure to UV rays, to a substrate, and also to a process for manufacturing a transistor comprising at least one step of performing such a process.
2. Discussion of Background Information
In many fields, it is necessary to connect together a layer made of a polymeric material with a substrate made of another polymeric or metallic material or comprising metallic or even semiconductive regions.
A polymeric material is a material that is capable of polymerizing, and is formed from the repeated sequence of the same unit, the monomer, the monomers being connected together via covalent bonds.
To obtain a film made of a polymeric material that is resistant either to mechanical stresses, or to chemical stresses, or to both, a process of crosslinking such a polymeric material is used. For certain materials, the crosslinking may be performed by exposure to UV rays.
In all cases, during crosslinking, the polymeric material passes from the viscoelastic state to the solid, rigid, elastic and unmeltable state.
Moreover, the adhesion of a material to the surface of another material may be defined as a set of physical and/or chemical phenomena that arise when two surfaces are placed in contact.
Adhesion is linked to adhesiveness, i.e. the ability to create forces of interaction between the two surfaces, at the surface state, i.e. the specific surface area, the porosity, the active sites and the pollution, of the two surfaces to be adhered together, and of the wettability of these two surfaces, i.e. their ability to create a mutual contact.
It is more particularly a matter of forces of adhesion, i.e. forces of cohesion of the materials, for example by creation of interatomic bonds.
These interatomic bonds may be intramolecular, as in the case of covalent chemical bonds, or intermolecular, as in the case of weak physical bonds.
The problem of improving the adhesion of a layer made of a polymeric material to the surface of a substrate made of a different material arises most particularly during the manufacture of a transistor.
Specifically, a transistor is generally composed of a substrate made of an organic semiconductive material, for example a modified pentacene, for example triisopropylsilyl pentacene (TIPS-pentacene), as described by Park et al. in “Solution-Processed TIPS-Pentacene Organic Thin-Film-Transistor Circuits, IEEE Electron Device Letters, Vol 28., No. 10, October 2007”, or perylene, or of an inorganic semiconductive material such as silicon, germanium or gallium arsenide.
Onto this substrate are then deposited source and drain electrodes and also a grille, which are generally made of metal, more particularly of aluminum, gold, copper or silver, and form metallic regions on the surface of the substrate.
These metallic regions are deposited according to the desired pattern onto the surface of the substrate, generally via a photolithographic process.
In such a process, a layer of a material that can be crosslinked by exposure to UV rays, but not polymerized, is deposited onto the desired surface of the substrate.
A mask comprising openings with the desired pattern is deposited onto this surface and the whole is exposed to UV rays.
The exposed regions of the polymeric material, that can be crosslinked by exposure to UV rays, are crosslinked. In the other regions, the material remains in nonpolymerized and noncrosslinked form.
The nonpolymerized and noncrosslinked regions are then removed by means of a solvent and the metal is deposited in the laid-bare regions of the surface of the substrate.
However, in such a process, after exposure to UV rays, and during the removal of the desired regions of polymeric material that remain noncrosslinked, the regions of crosslinked polymeric material detach from the surface of the substrate due to its lack of adhesion, which causes the solvent to attack this material by lifting it.
In concrete terms, the solvent intermixes between the substrate and the layer of UV-crosslinkable polymeric material, which leads to a deposition of material not in accordance with the desired pattern.
It is also necessary, during the manufacture of a transistor, to deposit an insulating material onto the surface of the semiconductive material optionally already comprising metallic regions.
The same photolithographic process as previously is repeated and the same problems of lack of adhesion of the polymeric material that can be crosslinked by exposure to UV rays to the surface of the metal and of the semiconductive material once again arise.